This invention relates to an asymmetric gas separation membrane which exhibits improved gas separation selectivity and the process to produce such an improved asymmetric gas separation membrane. In another aspect, the invention relates to processes utilizing such membranes for improved selectivity in the separation of at least one gas from a gaseous mixture by permeation.
The separating, including upgrading of the concentration of at least one selective gas from a gaseous mixture, is an especially important procedure in view of the demands and the supplies of chemical feedstocks. Frequently these demands are met by separating one or more desired gases from gaseous mixtures and utilizing the gaseous products for processing. Applications have been made employing separation membranes for selectively separating one or more gases from gaseous mixtures. To achieve selective separation, the membrane exhibits less resistance to transport of one or more gases than of at least one other gas of the mixture. Thus, selective separation can provide preferential depletion or concentration of one or more desired gases in the mixture with respect to at least one other gas and therefore provide a product having a different proportion of the one or more desired gases to the at least one other gas than the proportion in the mixture. However, in order for selective separation of the one or more desired gases by the use of separation membranes to be technically feasible the membranes must satisfy several criteria so that the use of the separation procedure has utility. For instance, the membranes must be capable of withstanding the conditions to which they may be subjected during the separation operation. The membranes must also provide an adequately selective separation of one or more desired gases at a sufficiently high flux, that is, permeation rate of the permeate gas per unit surface area. Thus, separation membranes which exhibit a high flux but low selectivity separation are unattractive as they require large separating membrane surface area. Similarly, separation membranes which exhibit adequate selectivity but undesirable low fluxes are also unattractive as they require large separating membrane surface area. Furthermore, membranes which are not capable of maintaining the desired performance of flux and selectivity over an extended period of time in adverse operating environments are likewise undesirable. Adverse environmental conditions include extremes of temperature, pressure and concentration of chemical contaminants. Accordingly, research has continued to develop gas separation membranes which provide improved performance regarding the separation of one or more desired gases for an extended period of time under adverse environmental conditions.
The passage of gas through a membrane can proceed through pores, i.e. continuous channels for fluid flow and communication with both feed and exit surfaces of the membrane (which pores may or may not be suitable for separation by Knudsen flow or diffusion); in another mechanism, in accordance with current views of gas separation membrane theory the passage of a gas through the membrane may be by interaction of a gas with a material of the membrane. In this latter postulated mechanism, the permeability of a gas through a membrane is believed to involve the solubility of the gas in the membrane material and the diffusion of the gas through the membrane. The permeability constant for single gas is presently viewed as being the product of the solubility and diffusiveness of the gas in the membrane. A given membrane material has a particular permeability constant for passage of the given gas by the interaction of the gas with the material of the membrane. The rate of permeation of the gas, i.e. flux through the membrane is related to the permeability constant, but is also influenced by variables such as the membrane thickness, the physical nature of the membrane, the partial pressure differential of the permeate gas across the membrane, the temperature and the like.
In general, efforts have been directed at providing material of a gas separation membrane in as thin a form as possible in view of the low permeabilities of most membrane materials in order to provide adequate flux while providing a membrane as porefree as possible such that gases are passed through the membrane by interaction with the material of the membrane. One approach for developing separation membranes suitable for gaseous systems has been to provide composite membranes having the thinnest possible superimposed membranes supported on an anisotropic porous support where the superimposed ultrathin membrane provides the desired separation. The superimposed membranes are advantageously sufficiently thin, i.e. ultrathin, to provide reasonable fluxes. The essential function of a porous support is to support the superimposed membrane. Suitable supports are desirably porous to provide low resistance to permeate passage after the superimposed membrane has performed its function of selectivity separating the permeate from the feed mixture. Klass et al, U.S. Pat. No. 3,616,607; Stancell et al, U.S. Pat. No. 3,657,113; Wasuda, U.S. Pat. No. 3,775,303; and Browall, U.S. Pat. No. 3,980,456 exemplify gas separation membranes having superimposed thin membranes on a porous support.
Such composite membranes for gas separations have not been without problems. For instance, Browall discloses that in the fabrication of composite membranes of ultrathin films, fine particles, particles below about 3,000 angstroms in size, may be deposited under or between preformed ultrathin membrane layers and because of their large size in comparison to the ultrathin membranes, puncture the ultrathin membranes. Such breaches reduce the selectivity and thus the effectiveness of the membrane. The Browall patent discloses applying a preformed organopolysiloxane-polycarbonate copolymer sealing material over the ultrathin membrane to cover the breaches caused by the fine particles. Browall also discloses employing a preformed layer of the organopolysiloxane-polycarbonate copolymer between the ultrathin membranes and the porous polycarbonate support as an adhesive. Thus, the composite membranes of Browall are complex in materials and techniques of construction.
A major improvement in gas separation membranes is disclosed by Henis et al in U.S Pat. No. 4,230,463 which pertains to particular multicomponent membranes for gas separations comprising a coating in contact with the porous separation membrane wherein the separation properties of the multicomponent membranes are principally determined by the porous separation membrane as opposed to the material of the coating. Such multicomponent membranes for the separation of at least one gas from a gaseous mixture can exhibit a desirable selectivity and still exhibit a useful flux. Moreover, such multicomponent membranes for gas separation can be fabricated from a wide variety of gas separation membrane materials which are advantageous for a given gas separation. The desired combination of flux and selectivity of separation can be provided by the configuration and methods of preparation and combinations of the components. For instance, material having high selectivity of separation but a relatively low permeability constant can be utilized to provide multicomponent membranes having desired permeation rates and desired selectivity of separation through utilization of a porous substrate which contributes to the overall separation efficiency of the multicomponent membrane.
Despite such advances in gas membrane separations, it would be advantageous to construct any membrane, including such multicomponent membranes, from materials that have reasonably good flux and substantially improved selectivity of separation while still remaining stable in use. That is, the membrane should have high structural strength, toughness, abrasion and chemical resistance to remain functional under extremes of temperature and differential pressure. In this regard such membranes are often required to operate at pressure differentials across the membrane of at least 2100 or 3500 KPa or higher, for instance up to 13800 KPa or ever higher. In general, operation environments require membrane material which is resistant to hydrocarbons, ammmonia, water, and acid gases such as carbon dioxide and hydrogen sulfide and the like. Such chemicals may have a tendency to dissolve or plasticize the material of the gas separation membranes resulting in deterioration of the membrane or densification of an otherwise asymmetric structure.
Research efforts continue in the field of gas separation membrane technology to reach economic gas separation performance utilizing asymmetric membranes of materials which have intrinsically high separation selectivity for gases such as hydrogen, carbon dioxide and the like. Attempts to eliminate surface porosity of these membrane materials in an asymmetric state by using either spinning or post treatment techniques have generally resulted in membranes, particularly hollow fiber membranes, that have poor performance gas separations either in low flux or low selectivities or both. With hollow fiber gas separation membranes spun from polymeric materials having high intrinsic selectivities for gas separations such as polyphenylene oxides, substituted polyphenylene oxides, polyimides, polyamides, polysulfones, polyethersulfones, cellulose esters, and the like, these treatments have produced modified fibers with uncoated separation properties of interest compared to those of multicomponent coated polysulfone fibers. We have found that treatment of flat or porous hollow fiber asymmetric gas separation membranes with Lewis acids offers a productive and simple route to uncoated gas separation membranes having desirable separation properties of commercial utility levels.